From Iron Curtain: The Crushing of Eastern Europe, 1944 - 1956, by Anne Applebaum
"Stalin is known to have favored promoting people who had some deep character flaw or secret, supposedly because he liked to have extra means of controlling his subordinates. Since Stalin had little faith in Polish communists in general, he might well have preferred a possible collaborator like Bierut to a true believer like Ulbricht. Anyone can lose their faith in communism, but blackmail is forever."
From Chapter 3, Communists